1. Field of the Invention
This invention relates generally to semiconductor optical amplifiers. More particularly, it relates to lasing semiconductor optical amplifiers used in combination with other optical elements to create a reconfigurable optical add drop multiplexer (OADM).
2. Description of Related Technologies
Fiber optic communications systems transmit information optically at high speeds over optical fibers. A typical communications system includes a transmitter, an optical fiber, and a receiver. The transmitter incorporates information to be communicated into an optical signal and transmits the optical signal via the optical fiber to the receiver. The receiver recovers the original information from the received optical signal. These systems are well adapted to transmit information at high speeds from one location to another.
However, efficient point-to-point transmission alone is not sufficient to construct a practical network. For example, a fiber running from New York to San Francisco may be efficient for transporting traffic from New York to San Francisco, but does not help much in transporting to/from Chicago, for example. To add Chicago to the New York-San Francisco route, traffic must be able to be added or dropped from the stream between New York and San Francisco.
An add-drop multiplexer (ADM) is the device which accomplishes this function. Many current ADMs are optical-electrical-optical, meaning that an incoming optical signal is converted to electrical form, the add-drop functionality is implemented electrically, and the resulting signals are then converted back to optical form. The two optical-electrical conversions add extra complexity and, strictly speaking, are unnecessary since the input and output signals are both optical.
Hence, ADMs which are entirely optical (in the sense that the signal remains in optical form while traveling through the ADM) are desirable. For example, in a wavelength division multiplexed (WDM) system, an optical add drop multiplexer (OADM) might work in the following manner. An optical signal carrying a wavelength division multiplexed (WDM) optical signal is input into a wavelength division multiplexer that demultiplexes the WDM optical signal into N single wavelength signals. The OADM is configured with N inputs, N outputs, N add inputs and N drop outputs. Each of the N single wavelength signals is coupled to an input to the OADM. As the N optical signals propagate through the OADM, each channel (i.e., each wavelength) can be dropped, added or passed through the OADM as needed. Dropping a channel means that channel is redirected by the OADM to the drop outputs. Conversely, adding a channel means that channel is received at one of the N add inputs and the OADM directs it to one of the OADM output ports. A pass through occurs when a channel is directed by the OADM from one input to an output. The N optical signals at the outputs of the OADM are wavelength division multiplexed back together into a single WDM optical signal. This signal is then forwarded out over the optical communications system.
OADMs may be either fixed or reconfigurable. In a fixed OADM, one or more of the channels are always dropped and/or added in the OADM. Thus, the path traveled by the particular optical signal is fixed based on which channel it occupies. In a reconfigurable OADM, switching elements inside the OADM allow each input signal to be dynamically added, dropped or passed through the OADM. Reconfigurable OADMs are preferred due to their increased functionality and flexibility in changing the topology of a network.
FIG. 1 is an illustration of a fixed OADM 100. A fixed OADM is configured such that N wavelength division multiplexed (WDM) channels are input into OADM 100 and M channels are dropped and added while the remaining N-M channels pass through OADM 100. In this implementation, a single WDM optical signal containing four optical channels enters wavelength division demultiplexer 125 from optical fiber 115. Wavelength division deumultiplexer 125 demultiplexes the WDM signal into four single wavelength optical signals 110A-D. Incoming channels 110C and 110D are dropped to outputs 120C and 120D, respectively. Their outgoing counterparts are added from inputs 130C and 130D, respectively. The other two channels 110A and 110B simply pass through OADM 100. As one can see from the figure, the paths traveled by the various channels are fixed.
FIG. 2 is an illustration of another fixed OADM 200 wherein the switching fabric is a combination of circulators and a wavelength grating. As illustrated, three port circulator 250 is coupled to input 210 of OADM 200. Circulator 250 is also coupled to fiber Bragg grating 260 and drop output 220. Three port circulator 270 is also coupled to grating 260 and is coupled to output 240 and add input 230.
OADM 200 works in the following way. A WDM optical signal enters OADM 200 through input 210. The optical signal passes through circulator 250 and encounters grating 260. Grating 260 reflects a desired wavelength signal back to circulator 250 and passes the other wavelengths of the optical signal to circulator 270. The reflected signal is directed to drop output 220 by circulator 250. The remainder of the optical signal that was passed to circulator 270 is directed to output 240. However, an optical signal of the wavelength that was dropped can also be added to the remainder of the optical signal at circulator 270. Such a signal enters OADM 200 through add input 230. This signal then enters circulator 270, reflects off grating 260 and is combined with the remainder of the original optical signal. This new optical signal is then sent to output 240.
In addition to the two devices described above, an OADM can also be based on a crossbar switch. There are a number of devices and techniques that can implement (or attempt to implement) the basic switching of a crossbar. For example, various groups are attempting to develop optical crossbar switches based on MEMS (e.g., micro-mechnical mirrors), BUBBLES, liquid crystal (mirrors), LiNO3, or thermal optic switches. However, each of these devices has a loss associated with it when performing the switching function. This aspect of these devices makes their use less attractive in OADMs, particularly since switching may be cascaded in larger crossbars, thus compounding the overall loss through the crossbar. In addition, many of these approaches also suffer from other disadvantages, such as slow switching speed, large size, requiring complicated electronics, excessive intersymbol crosstalk and/or excessive crosstalk between different channels (e.g., WDM channels).